Sequential color display system typically use a color wheel to filter a white light beam into a light beam having a sequence of at least three colors. The light beam is spatially modulated to produce an image. The modulator is synchronized with the color wheel so that a series of monochromatic images are formed. The persistence of the viewer's eye is used to integrate the monochromatic images and provide the perception of a full color image.
The use of a color wheel has several disadvantages. The primary disadvantage is that only about one-third of the light produced by the light source is used. The other two-thirds are filtered from the light beam by the color wheel and converted to heat by absorption in the system. Because only one-third of the light is used, a higher intensity light source is required to produce a given image brightness. The higher intensity light source requires more energy to operate and creates a much larger thermal load on the display system. Cooling the light source and the thermal load from the unused light requires a significant amount of energy which itself produces heat due to the inefficiency of the system. Additionally, the cooling fans and air plenums require a significant amount of space in the display system and create a significant amount of noise.
Color wheels are also inefficient when used with a field addressed modulator such as a micromirror device. The transitions between the various color filters require the modulator to be turned off to avoid creating mixed color images. Some display systems use the transition or spoke light to form secondary color images or white or gray scale images, but these systems require a significant amount of processing power to be devoted to the calculations necessary to utilize the spoke light. Scrolling color systems image the multiple colors of the color wheel onto the modulator and provide single-color image data to the modulator elements on either side of the filter transitions. The image data is changed as the filter transition sweeps across the face of the modulator.
Unfortunately, typical color wheels produce pie-shaped color segments as they sweep across the face of the modulator. Modulators such as the typical micromirror device group several rows of modulator element in a reset group and operate the entire reset group in concert. The pie-shaped segments create inefficiencies in this type of modulator since the entire reset group is turned off when the spoke sweeps across any portion of the group. Thus, pie-shaped filter segments often require two or more reset groups to be turned off for each spoke.
Color wheels that minimize the tilt of the filter transition relative to the rows of modulator elements, such as the spiral of Achimedes color wheels or barber pole style cylindrical drums are desired. These color wheels maximize light passing through the color wheel that is able to be used by a rasterized modulator. The combination of relatively small filter interfaces lined up with the grouping of the modulator elements and color filter segments small enough to allow one of each color to simultaneously be in the light path, enables the use of the sequential color filter in a highly efficient scrolling light recycling system.
What is needed is an improvement of the existing color wheels that minimizes the load on the display system power and cooling resources while providing more efficient optical operation with row-based spatial light modulators.